A coating type magnetic recording medium consists of a nonmagnetic support such as polyethylene terephthalate, and a magnetic layer formed by coating a magnetic coating solution comprising ferromagnetic powder-homogeneously dispersed in a binder solution of a resin. As the ferromagnetic powder, acicular ferromagnetic alloy powders, e.g., γ-Fe2O3 and Fe—Co have been conventionally used, but for the improvement of recording density, ferromagnetic powders using superfine particle magnetic powders of hexagonal ferrite are developed and partially put to practical use in recent years.
In the field of the magnetic disc, along with the increase in the amount of data to be dealt with, and the increase of the capacity of floppy discs has been demanded.
In the field of the magnetic tape also, with the prevalence of office computers, such as minicomputers, personal computers and work station, magnetic tapes for recording computer data as external storage media (so-called backup tapes) have been vigorously studied. For putting magnetic tapes for such uses to practical use, the improvement of recording capacity has been strongly demanded conjointly with the miniaturization of a computer and the increase of information processing performance for achieving high capacity recording and miniaturization.
Magnetic heads working with electromagnetic induction as the principle of operation (an induction type magnetic head) are conventionally used and spread. However, the magnetic heads of this type are approaching their limit for use in the field of higher density recording and reproduction. That is, it is necessary to increase the number of winding of the coil of a reproduction had to obtain larger reproduction output, but when the winding number is increased, the inductance increases and the resistance at high frequency heightens, as a result, the reproduction output lowers. In recent years, reproduction heads that work with MR (magneto-resistance) as the principle of operation are proposed and get to be used in hard discs. As compared with the induction type magnetic disc, several times of reproduction output can be obtained by using MR head. Further, since an induction coil is not used in MR head, noises generated from instruments, e.g., impedance noises, are largely reduced, therefore, it becomes possible to obtain a great S/N ratio by lowering the noise coming from magnetic recording media. In other words, good recording and reproduction can be done and high density recording characteristics can be drastically improved by lessening the noise of magnetic recording media hiding behind the instruments.
Abrasives are generally added to a magnetic layer for ensuring running durability conjointly with the improvement of electromagnetic characteristics. For instance, it is well known to use abrasives having high Mohs' hardness, typically α-alumina, in a magnetic layer.
For example, JP-A-2001-101647 (The term “JP-A” as used herein refers to an “unexamined published Japanese patent application”.) proposes a magnetic recording medium comprising a nonmagnetic support having a nonmagnetic layer containing a binder, and a magnetic layer having a thickness of 0.5 μm or less provided on the nonmagnetic layer after being coated and dried, wherein the abrasive contained in the magnetic layer has a Mohs' hardness of 5 or more, and a relative abrasive velocity of 200 or more, and value (A) of the relational expression (A)=(D)/(T) of the average particle size (D) of the abrasive and the magnetic layer thickness (T) is from 0.55 to 1.20.
In a coating type magnetic recording medium, it is thought that when magnetic powder is made finer and magnetic layer is made thinner with the trend of higher density, it is desired to make abrasive finer to ensure smoothing of a magnetic layer, but as a matter of fact when abrasive is made finer, the relative abrasive velocity lowers and the compatibility with running durability is difficult.
In particular, when an MR head is used in the reproduction of a magnetic recording medium, the influence of abrasive becomes great and when abrasive force is increased to maintain running durability, it is accompanied by reduction of S/N ratio and abrasion of a head.